Desirably, a lighting device that stably lights up LEDs (light emitting diodes) serving as loads should be under constant current control, which outputs a constant output electric current for any load. The reason is as follows: the voltage-current characteristics of an LED element have a non-linear feature in which an electric current starts flowing suddenly at a certain applied voltage or higher, a forward voltage does not substantially change while an electric current near a rated current value is flowing, and light output basically depends on a value of an electric current that is flowing. By providing control so that a constant electric current flows through the LED element for any output voltage, the constant current control can reduce variations in light output in the case where there are variations in lighting voltage due to the individual difference among the LED elements. Also, even when loads having different rated lighting voltages are connected or even when the number of loads connected in series is changed, the constant current control can accommodate various connection styles because a constant electric current can be passed through the loads.
Conventionally, various lighting devices have been proposed that suppress the variations in light output of LED elements based on the constant current control described above (for example, see Japanese Unexamined Patent Application Publications No. 2012-109141 and No. 2010-40509).
In Japanese Unexamined Patent Application Publication No. 2012-109141, it is contrived how to reduce variations in output electric current due to voltage ripples of a DC power source.
In general, a buck converter is operated by boundary current mode (BCM) control and peak current control, making it possible to pass a constant electric current through an LED connected to the buck converter regardless of a forward voltage of the LED. Herein, the BCM control and the peak current control turn OFF a switching element when an electric current value detected by a current detection circuit reaches a predetermined value, and turn ON the switching element when release of predetermined energy from an inductor is detected, in the buck converter. In such a BCM control, an average output electric current is half the electric current peak value. In the peak current control, the switching element is turned OFF when the electric current flowing through the inductor reaches a peak current reference value Iref. In this way, the peak value of the electric current flowing through the inductor is matched to the reference value Iref, making it possible to keep the output electric current at a constant value (½ of the current reference value Iref) regardless of the output voltage. However, components constituting the buck converter have a delay time (for example, a delay time of a detection operation circuit, a signal output delay time of a driver IC, a drive delay time of the switching element, etc.). Accordingly, there occurs a delay from when the electric current flowing through the inductor reaches the peak current reference value Iref until the switching element is turned OFF. When an input voltage of the buck converter ripples, such a delay time causes an actual peak value Ipeak of the electric current flowing through the inductor to be larger than the reference value Iref, so that the light output is also varied. In response to the above problem, Japanese Unexamined Patent Application Publication No. 2012-109141 detects a voltage corresponding to the input voltage of the buck converter using a secondary winding of the inductor so as to correct the peak current reference value Iref.
On the other hand, Japanese Unexamined Patent Application Publication No. 2010-40509 contrives, in a lighting device including a plurality of outputs (output terminals), a circuit in which electric currents from the individual outputs are equated with one another. With respect to a common electric current reference value REF, individual buck converters calculate an average electric current flowing through switching elements, and perform feedback control so as to match the average electric current with the reference value. In other words, an electric current flowing through the switching element of each buck converter is monitored, and the difference between a monitored electric current Isen and the reference electric current REF is calculated by an error amplifier. Then, by calculating a logical sum of an output of the error amplifier and a sawtooth waveform (a RAMP waveform), a duty ratio of a drive signal of the switching element is regulated so that an average value of the monitored electric current Isen and the REF are equated with each other during a period in which the switching element is ON. In such a control, the constant current control is performed normally by a continuous current mode (CCM).